Since March there has been a massive rally in all sorts of risky assets – equities, oil, energy and commodity prices – a narrowing of high-yield and high-grade credit spreads, and an even bigger rally in emerging market asset classes (their stocks, bonds and currencies). At the same time, the dollar has weakened sharply , while government bond yields have gently increased but stayed low and stable.

This recovery in risky assets is in part driven by better economic fundamentals. We avoided a near depression and financial sector meltdown with a massive monetary, fiscal stimulus and bank bail-outs. Whether the recovery is V-shaped, as consensus believes, or U-shaped and anaemic as I have argued, asset prices should be moving gradually higher.

But while the US and global economy have begun a modest recovery, asset prices have gone through the roof since March in a major and synchronised rally. While asset prices were falling sharply in 2008, when the dollar was rallying, they have recovered sharply since March while the dollar is tanking. Risky asset prices have risen too much, too soon and too fast compared with macroeconomic fundamentals.

So what is behind this massive rally? Certainly it has been helped by a wave of liquidity from near-zero interest rates and quantitative easing. But a more important factor fuelling this asset bubble is the weakness of the US dollar, driven by the mother of all carry trades. The US dollar has become the major funding currency of carry trades as the Fed has kept interest rates on hold and is expected to do so for a long time. Investors who are shorting the US dollar to buy on a highly leveraged basis higher-yielding assets and other global assets are not just borrowing at zero interest rates in dollar terms; they are borrowing at very negative interest rates – as low as negative 10 or 20 per cent annualised – as the fall in the US dollar leads to massive capital gains on short dollar positions.

Let us sum up: traders are borrowing at negative 20 per cent rates to invest on a highly leveraged basis on a mass of risky global assets that are rising in price due to excess liquidity and a massive carry trade. Every investor who plays this risky game looks like a genius – even if they are just riding a huge bubble financed by a large negative cost of borrowing – as the total returns have been in the 50-70 per cent range since March.